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Measuring The Liquid Level By Using Differential Pressure Transmitter
Nov 28, 2018


Measuring the liquid level by using differential pressure transmitter



 Differential pressure transmitter is a kind of thermal measuring instrument which is very common in industrial production. It mainly acts to measure the pressure of liquid medium, liquid level gauge, level gauge, level transmitter and liquid level. Transmitter, capacitive level gauge, capacitive liquid level gauge, capacitive level transmitter, capacitive liquid level transmitter, level switch, liquid level switch, capacitance level gauge, capacitance level meter, Liquid level meter, level meter, capacitor level transmitter, capacitor level transmitter, RF capacitive level switch, RF capacitive level switch, capacitive level switch, capacitive level switch, tuning fork Physical parameters such as level switch and flow rate. At present, in the industrial automation production equipment, the application range of the differential pressure transmitter is more extensive, but as a general-purpose measuring instrument, the operation failure often occurs. If there is a problem in production, we need to be able to solve it in time. If it cannot be handled quickly, it will certainly affect the normal operation of production to a certain extent, and some serious failures may even endanger personal safety. This article is a long-term experience accumulated by Runzhong Instrument Technology Co., Ltd., briefly discussing the application of differential pressure transmitter in liquid level measurement.

Measuring the liquid level by using differential pressure transmitter

 

First, the working principle of differential pressure transmitter


  Differential pressure transmitters are commonly used to measure the level of liquid in a closed container, using the pressure difference created by the gravity of the liquid to measure the level of liquid in the container. The high-pressure side measuring tube (above the figure) is always filled with water due to steam condensation, keeping the pressure constant, while the low-pressure side measuring tube (located below the figure) forms a communication device with the container, and the pressure changes with the liquid level in the container. Linear change.

  Let △P be the differential pressure signal received by the transmitter, P0 is the internal pressure of the container, P+ is the pressure on the positive pressure side of the transmitter, P- is the pressure on the negative pressure side of the transmitter; ρ is the density of the liquid in the container; It is the acceleration of gravity; h1 is the height of the process zero point to the upper pressure port of the container; h2 is the process level of the container; h is the height of the transmitter to the zero point of the process liquid level.

Then there are:

 P+=P0+ρgh1+ρgh

P-=P0+ρgh2+ρgh

△P=P+-P-=ρgh1-ρgh2

  When the liquid level changes from h2=0 to h2=h1, the differential pressure measured by the differential pressure transmitter changes from the maximum value to ΔP=0. By setting the transmitter, the output current is changed from 4 mA to 20 m.

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Second, the transmitter zero setting


  When the differential pressure transmitter measures the liquid level, the setting of the zero position is a very important part. When the high pressure (H) side and low pressure (L) side of the transmitter are connected with the high pressure side and low pressure side of the local measuring cylinder, the high pressure side pressure guiding tube is always in a full water state, and the pressure is measured at the high pressure end of the transmitter. For P+kPa, the low-pressure side of the transmitter is connected to the low-pressure side pressure guiding tube, and the measured pressure is P-kPa, then the actual differential pressure measured by the transmitter is (P+-P-) kPa. When the liquid level of the container is the lowest, the differential pressure value is the largest, corresponding to the internal setting of the transmitter LRV, that is, the zero position of the transmitter. At this time, the output current of the transmitter is 4 mA, and when the liquid level of the container is the highest, the differential pressure value is 0. Corresponding to the internal setting of the transmitter URV, which is the fullness of the transmitter, the transmitter output current is 20mA. When the high pressure (H) side and low pressure (L) side of the transmitter are opposite to the high voltage side and low voltage side of the local measuring cylinder, the internal setting of the transmitter needs to be modified: the LRV of the transmitter is set to ( P--P+) kPa (this difference is negative), that is, regardless of how the transmitter and the pressure guiding tube are connected, the fullness of the transmitter corresponds to the full water level of the measuring container, and the differential pressure is always 0, ie The transmitter's full-scale URV is 0kPa and the output current is 20mA. When the high voltage side of the transmitter is connected to the high voltage side of the pressure guiding tube, the transmitter zero position LRV is set to the maximum differential pressure value. When the high voltage side of the transmitter is connected to the low pressure side of the pressure guiding tube, the transmitter zero position The LRV is set to the negative of the maximum differential pressure value.


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Third, the installation requirements of differential pressure transmitter


  In order to accurately measure the liquid level, in addition to the correct selection and verification of the differential pressure transmitter, it must also be noted that the installation of the entire system meets the requirements. The indication value of the transmitter sometimes does not reflect the actual parameters of the measured medium, because the measurement system itself will produce errors. System installation requirements include: the opening position of the pressure tap, the proper laying of the connecting conduit, and the mounting position of the transmitter.

  First of all, the pressure tap should be in a region where the fluid flow is smooth and there is no eddy current, and the process parameters should be ensured in the process. For example, when the differential pressure transmitter is used to measure the water level of the boiler drum, the actual water level of the steam drum is different in the axial and radial directions of the steam drum, generally in the axial direction, the middle water level is high, and the water level on both sides is low; The side of the lowering tube is higher on the denser side. The condenser liquid level transmitter of a power plant turbine is close to the condensate pump inlet due to the position of the measuring point. When the condensing pump is running, it causes the water surface to sink at the point of the point, which causes the transmitter to be significantly lower. The position of the measuring cylinder is moved to a distance from the inlet of the condensing pump, and the condenser water level transmitter is consistent with the actual water level indication on the spot to ensure safe and stable operation of the condenser. Secondly, when measuring the liquid level, the differential pressure measured by the differential pressure transmitter is relatively small, generally between several kPa and 100 kPa, so the entire measurement system has a great influence on the measurement accuracy. As shown in Figure 1, when installing the pressure guiding tube, the horizontal section of the pressure guiding tube should have a certain inclination, and the inclination should be as large as possible to avoid accumulation of liquid inside the pressure guiding tube, resulting in inaccurate measurement. When the transmitter range is small, the transmitter output will fluctuate. In addition, when the transmitter is put into operation, the air bubbles in the liquid column of the pressure guiding tube should be drained as much as possible. These accumulated gases will affect the accuracy of the measurement. It is also conceivable to arrange the exhaust device at the highest point of the upper bend of the pressure guiding tube.



Fourth, the transmitter zero migration


  When the differential pressure transmitter measures the liquid level, if the positive and negative pressure chambers of the differential pressure transmitter are at the same level as the pressure point of the container, no migration is required. In practical applications, the transmitter may not be at the same level as the pressure point for the installation location and ease of maintenance; for example, the measured medium is a highly corrosive or heavy viscosity liquid. The medium cannot be directly introduced into the transmitter. The isolation liquid tank must be installed and the pressure signal should be transmitted with the isolation liquid to prevent the transmitter from being corroded. At this time, it is necessary to consider the influence of the liquid column of the medium and the spacer on the measured value of the transmitter. When the installation position of the transmitter is often not at the same level as the minimum liquid level, in order to correctly indicate the height of the liquid level, the differential pressure transmitter must do some technical processing, ie migration. Migration is divided into no migration, negative migration, and positive migration. The so-called "migration" of the transmitter is to move the measurement range while the transmitter is in the same range. Usually, the measurement starting point is moved below the reference point “0”, which is called negative migration; the measurement starting point is moved above the reference point “0”, which is called positive migration. Taking a differential pressure transmitter of 30 kPa range as an example, the measurement range is 0 to 30 kPa when there is no migration, the measurement range is 30 to 60 kPa when the positive migration is 100%, and the measurement range is -30 to 0 kPa when the negative migration is 100%. The measurement range is -15 to +15 kPa when the negative migration is 50%.

  In the actual operation, first determine the range of the differential pressure transmitter. After calibration, use the migration screw to adjust the measurement starting point or full-range output to the corresponding position or use the hand-operator to directly input the migration amount. For example, if you need to measure the differential pressure of -30~0kPa, the range is 30kPa. When the transmitter is calibrated, the negative pressure chamber is pressurized by 30kPa, and the differential pressure transmitter zero knob is adjusted to make the output 4mA; then, the negative pressure The chamber is not pressurized, adjust the differential pressure transmitter range knob until the output is 20mA. If the hand-operator is used, set the transmitter's LRV to -30kPa and the URV to 0kPa. The measurement range of the differential pressure transmitter is equal to the sum of the range and the migration amount, that is, the measurement range = range range + migration amount. Therefore, the essence of positive and negative migration is to change the upper and lower limits of the range of the differential pressure transmitter, and the size of the range is unchanged. According to the principle of measuring the positive and negative migration of liquid level by differential pressure transmitter, in practical application, the measurement method of liquid level can be improved according to the use condition of the instrument, the process condition of the production device and the surrounding environment. The capacitive transmitter produced by Runzhong Instrument Technology Co., Ltd. has high measurement accuracy and simple debugging. The transmitter itself may have a very small fault. When working for a period of time, if the indication deviation is large, it may not be normal. The problems are mainly caused by blockage or leakage of high and low pressure side pressure guiding tubes or three valve groups, change of migration amount, zero drift, loss of isolation liquid, etc., or the transmitter works in a negative pressure environment, and the transmitter measurement circuit The seal is not good and the outside air enters the measuring line. When the output value of the differential pressure transmitter is too large or too small, the pressure guiding tube can be disconnected from the three-valve assembly, the negative pressure side is connected to the atmosphere, and the differential pressure signal is sent to the transmitter by the hand pump. On the pressure side, observe whether the output of the transmitter is correct, that is, the output current is 4~20mA, and the error does not exceed 0.8mA. If the output of the transmitter is correct, it can be judged that the pressure guiding pipeline is blocked or leaked.


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